1. Field of the Invention
This invention relates to a two channel pseudo play-back apparatus for pseudo reproduction of a multi-channel audio output, and particularly relates to a two channel pseudo play-back apparatus in a personal computer for pseudo reproduction by two channel speakers of multiple channel audio outputs from, for example, DVD-ROMs.
2. Background Art
Conventionally, in a personal computer loaded with a reproducible sound board of sound or music, it was standard to use two channels for outputting an audio output of the medium for playing back.
Examples of reproducible mediums of sound or music in a personal computer include CD-ROM or CD-I (Compact Disc-Interactive). In the play-back of CD-I, there is a system which is capable of obtaining particular sound effect by processing audio signals for right and left channels, as disclosed in Japanese Patent Application, First Publication No. Hei 2-261213. This apparatus is provided with a function for producing particular sound effects using a four-channel mixing circuit and this apparatus comprises a digital audio processing portion containing a four-times over sampling type digital filter, and outputs two channel audio signals using the four channel mixing circuit by individually processing digital audio data for the L channel and the R channel.
Recently, attention has been attracted to an audio data signal system for digital video discs (hereinafter called DVD), which is a high efficiency coding system of AC-3 developed by Dolby Research Institute in the United States. This AC-3 is capable of playing back 5.1 channels of surrounding sounds. The number of channels in this AC-3 system is defined as two right and left front channels, one front center channel, two rear channels, and a low band channel (0.1 channel) for driving a super woofer.
For such audio data of 5.1 channels are reproduced by a personal computer with two audio output channels, two methods are known as described hereinafter.
A stereo play-back apparatus using the first method is described hereinafter with reference to FIG. 4. The first method comprises the steps of reading compressed digital audio data from a digital video disc-read only memory (DVD-ROM) 11 which stores the compressed audio data, and expanding the compressed data into 5.1 channel audio data by a decoder 12 of the AC-3, and storing expanded data temporarily in a buffer 13 composed of a plurality of buffers 131 to 136. Subsequently, among digital audio data stored in the buffer 13, digital audio data (FR and FL) for the left and right front channels are converted into analog signals by respective D/A converters constituted by a left D/A converter (L) 14 and a right D/A converter (R) 15, and thus converted analog signals are output into the audio signal output lines for playing back by left and right speakers 16 and 17.
A stereo play-back apparatus using the second method is described hereinafter with reference to FIG. 5. The same reference numerals are used in FIG. 5 for the same elements as those of FIG. 4, and explanations of the same elements are omitted. The stereo play-back apparatus using the second method comprises the same steps as those described referring to FIG. 4, that is, reading compressed digital audio data from the DVD-ROM, expanding the compressed data into digital signals for 5.1 channels using the AC-3 decoder 12, and storing the expanded data temporarily in the buffer 13. The steps are followed by mixing audio data for 3.1 channels containing the left front channel (FL), left rear channel (RL), the front center channel (FC), and a super woofer (SU) using a left mixing circuit (L) 21, converting the mixed data into analog data using a left D/A converter (L) 14, outputting the analog data into the left audio outputting line for play-back of the sound by a left speaker 16. Similarly, the audio data for 3.1 channels containing the right front channel (FR), right rear channel (RR), the front center channel (FC), and the super woofer (SU) are mixed using a right mixing circuit (R) 22, the mixed data are converted into an analog signal using a right D/A converter (R) 15, and the analog signals are output to a right outputting line for play-back using a right speaker 17.
Therefore, it is an object of the present invention to provide a pseudo stereo play-back apparatus with two channel speakers, which is capable of playing back a superior surround effect which is not obtainable using a usual two-channel play-back apparatus by processing the multiple channel audio outputs from a recording medium such as DVD-ROM for the two channel audio outputting apparatus.
Another object of the present invention is to provide a pseudo stereo play-back apparatus with two channel speakers, which is capable of playing back a superior surround effect which cannot be obtained with the usual two channel audio output apparatus by pseudo reproduction of the multiple channels by a two channel audio apparatus from a recording medium containing multiple channel audio outputs.
According to an aspect of the present invention, a pseudo multi-channel stereo play-back apparatus comprises: (a) a decoder for expanding compressed digital audio signals channels stored in a recording medium into digital audio signals for a plurality of channels; (b) a memory means for temporarily storing digital audio signals expanded by said decoder; (c) a phase control circuit for controlling phases of digital audio signals for specified channels which are a part of digital audio signals for all channels stored in said memory means; (d) first and second mixing circuits for mixing the digital audio signals after being transformed by said phase control circuit with other digital audio signals for other channels stored in said memory means; and (e) first and second A/D converters for converting respective outputs of the first and second mixing circuits into analog sound signals.
That is, this aspect of the present invention performs pseudo play-back of a surrounding sound by the steps of expanding the multi-channel digital audio signals stored in a memory, and storing the expanded signals in a memory. Subsequently, among stored signals for all channels, digital audio signals for the left and right rear channels and for the front center channel are subjected to the phase control by the phase control circuit. Further, other digital audio data for the left and right front channels are transferred to the mixing circuits as they are and after they are mixed with the first and second mixing circuits with the phase transformed signals for the rear channels and the front center channel, outputs of the first and second mixing circuits are converted by the first and second D/A converters, respectively, into analog audio signals. The analog audio signals from both converters are played back by two respective speakers located at both left and right positions. Consequently, the present apparatus performs pseudo play-back of a multi-channel surrounding sound with two speakers of a personal computer.
According to another aspect of the present invention, the pseudo multi-channel stereo play-back apparatus comprises: (a) a decoder for expanding compressed digital data stored in said memory device; (b) a memory means for temporarily storing digital data expanded by said decoder; (c) a phase control circuit for controlling phases of the digital audio signals for left and right rear channels and a phase for the front center channel among digital audio signals for all channels stored in said memory means; (d) first and second mixing circuits for mixing the phase transformed digital audio signals for left and right rear channels, digital audio signals for left and right front channels stored in said memory, and the phase transformed digital audio signal of the front center channel; and (e) first and second D/A converters for converting respective outputs of said first and second mixing circuits into analog sound signals.
The pseudo multi-channel stereo play-back apparatus of the second aspect of the present invention performs pseudo play-back of multi-channel surrounding sound by first expanding the multi-channel digital audio signals stored in a memory and storing the expanded signals in a memory. Among stored signals for all channels, digital audio signals for the left and right rear channels and for the front center channel are subjected to the phase transformation by the phase control circuit. Subsequently, the other digital audio data for the left and right front channels are transferred to the mixing circuits as they are and after they are mixed using the first and second mixing circuits with the phase transformed signals of the rear channels and the front center channel. The outputs of the first and second mixing circuits are then converted by respective first and second D/A converters into analog audio signals and these analog audio signals from both converters are played back by two speakers located at both left and right positions. Consequently, a multi-channel surrounding sound can be played back by pseudo reproduction using two speakers of the personal computer.
According to an other aspect of the present invention, the first and second mixing circuits of the pseudo multi-channel stereo play-back apparatus mixes digital audio signals for left and right front channels and left and right rear channels, giving each audio signal a space transmission characteristic which represents a play-back sound field and speaker characteristics.
In this aspect of the present invention, the first mixing circuit mixes digital audio signals for the left front and the left rear channels, giving each digital audio signal a space transmission characteristic which represents the play-back sound field and the speaker characteristics, and the second mixing circuit mixes digital audio signals for the right front and the right rear channels, giving each digital audio signal a space transmission characteristic which represents the play-back sound field and the speaker characteristics. Thereby, an audience at its position can hear a sound from both the left and right speakers with a deep and stereo feeling.
According to still an other aspect of the present invention, the digital audio signal for the front center channel, after it is subjected to the phase control processing, is output into both first and second mixing circuits.
That is, according to this aspect of the present invention, the digital audio signals for the front and rear channels are mixed with both first and second mixing circuits, so that the audio signal for the front center channel is localized in each sound and in each sound image of each left and right front and left and right rear channels to play-back by two speakers, which results in increasing the surrounding effect.
According to still an other aspect of the present invention, digital audio signals for the super woofer channel which is used for low frequency sound is directly input into both mixing circuits for mixing.
That is, digital audio signals for the super woofer channel for driving low frequency sounds are transferred and mixed in the first and second mixing circuits, which results in increasing the low frequency sound when playing back by two speakers.
According to still another aspect of the present invention, a pseudo multi-channel stereo play-back apparatus comprises: (a) a decoder for expanding compressed digital audio data stored in a memory into digital audio signals for a plurality of channels; (b) a memory means for storing digital sound data for each channel after expansion by said decoder; (c) a phase control circuit for controlling the digital audio signals for the left and right rear channels and the digital audio signals for the front center channel from among digital sound data stored in said memory means; (d) first and second mixing circuits for mixing digital audio signals for the left and right rear channels after being phase transformed, digital audio signals for the left and right front channels as stored in said memory, and digital audio signals for the front center channel after being phase transformed at respective mixing circuits; (e) first and second D/A converters for converting outputs of said first and second mixer circuits into respective analog audio signals.
That is, according to this aspect of the present invention, a pseudo multi-channel play-back apparatus performs pseudo multi-channel reproduction for playing back of a surrounding sound by first expanding the multi-channel digital audio signals stored in a memory by the decoder, and storing the expanded signals in a memory. Subsequently, among stored signals for all channels, digital audio signals for the left and right rear channels and for the front center channel are subjected to the phase control by the phase control circuit, and the remaining digital audio data for the left and right front channels are transferred as they are to the mixing circuits. After the transferred signals are mixed with the phase transformed audio signals by the first and second mixing circuits, outputs of these mixing circuits are converted by respective first and second D/A converters into analog audio signals. The analog audio signals from both converters are played back by two speakers located at both left and right positions as a pseudo reproduction of the multi-channel surrounding sound. That is, a multi-channel surrounding sound can be played back by a personal computer with two speakers.